This invention generally relates to the fabrication of micro-electromechanical switching (MEMS) devices that can be fully integrated into current state of the art semiconductor fabrication processes, and more particularly, for incorporating self-aligned spacers or stops to reduce stiction between a movable beam and an electrode.
Switching operations are a fundamental part of many electrical, mechanical and electromechanical applications. MEM switches have drawn considerable interest over the last few years. Products using MEMS technology are widespread in biomedical, aerospace, and communications systems.
Conventional MEMS typically utilize cantilever switches, membrane switches, and tunable capacitor structures as described, e.g., in U.S. Pat. No. 6,160,230 to McMillan et al., U.S. Pat. No. 6,143,997 to Feng et al., U.S. Pat. No. 5,970,315 to Carley et al., and U.S. Pat. No. 5,880,921 to Tham et al., MEMS devices are manufactured using micro-electromechanical techniques and are used to control electrical, mechanical or optical signal flows. Such devices, however, present many problems because their structure and innate material properties require them to be fabricated in manufacturing lines that are separate from conventional semiconductor processing, particularly, CMOS. This is usually due to the different materials and processes that are incompatible and, consequently, not able to be integrated in standard semiconductor fabrication processes.
Materials that are typically used in the manufacture of MEMS, such as gold, pose obvious problems for integrating devices directly to on-chip applications. Even the use of polysilicon, which is widely found in the literature, poses problems to conventional fabrication facilities. One problem is that the polysilicon deposition process requires temperature cycles that are higher than the melting point of metals. Another problem is that polysilicon deposition tools are typically unavailable for use in the back-end of the line (BEOL), i.e., wherein interconnect metals are processed. This problem is mainly caused because the tool sets designated as front-end of the line (FEOL), i.e., where the actual semiconductor devices are fabricated, are segregated from all the tool sets designated for the back-end of the line (BEOL). The two sets do not permit process crossovers from one to the other in order to prevent metallic contamination of the active devices.
Accordingly, there is a need for a process that is capable of providing MEMS devices using established BEOL materials coupled to processing that can be fully integrated so that devices can be manufactured either in conjunction with or as an add-on module to the conventional BEOL or interconnect levels.
In order to gain a better understanding of the present invention, a conventional MEM switch will now be described with reference to FIG. 1, which shows a cross-section view of a MEM switch having both ends of a deformable beam 1 anchored in dielectric 4. The lowest level consists of a dielectric material 5 containing conductive elements 2, 2a, and 3 which will be used subsequently to connect or form the various electrical portions of the device. The conductors referenced by numerals 2 and 2a provide an operating potential that causes the beam to deform. Conductor 3, which conducts the electrical signals is, in turn, connected to the beam when it is in operation.
In a typical implementation, deformable beam 1 is formed by depositing polysilicon over dielectric 4, e.g., SiO2, and the surrounding material is etched away leaving a raised structure, i.e., the beam suspended above the conductors that were previously formed or which, themselves, are made of polysilicon. Then, the device is subjected to electroless plating, usually with gold, that adheres to the polysilicon forming the conductive elements 1, 2, 2a and 3. The switch operates by providing a potential difference between the beam and electrodes 2 and 2a. This voltage generates an electrostatic attraction that pulls beam 1 in contact with electrode 3, thus closing the switch.
FIG. 2 is a cross-section view and planar view respectively of another prior art MEM device similar to the devices shown in FIG. 1, except that only one end of the beam 1 anchored in the dielectric 4. Herein, the control electrode is referenced by numeral 2 and the switching electrode by numeral 3. This device is constructed and operates in a similar fashion to the one previously described.
When fabricating MEMS devices, it is desirable to introduce certain limitations or stops to inhibit movable parts from coming into contact with certain surfaces. It may also be required to prevent or limit the movement of certain switching elements or at least portions of these. This is the case illustrated in the accompanying drawings, within which are shown how stops are used to fabricate RF capacitive switches. The invention addresses these problems in detail, as will be explained hereinafter.
Another significant problem plaguing present art micro-electromechanical contact switches resides in the electrodes tending to stick to one another upon contact, making it difficult to separate them in order to turn the switch off. This phenomenon, known as xe2x80x9cstictionxe2x80x9d is caused by the attraction at the microscopic level between atoms and molecules on the two surfaces. One solution is to ensure that when one contact plate is deflected to close the switch, the deflection creates a spring-like restorative force that naturally attempts to separate the contacts. If large enough, such a force can overcome stiction. However, the same force also implies that a large force must be generated to deflect the contact to close the device. In a switch wherein the deflection is electrostatic, this generally implies the need for a high control voltage beyond the 5V maximum that is required in, for instance, mobile handsets.
The stiction problem is not novel, and certain aspects of it have been described in the art. By way of example, in U.S. Pat. No. 5,772,902 to Reed et al., there is described a method for preventing adhesion of micro-electromechanical structures sticking to each other during fabrication. The structure described therein applies to micro-electromechanical systems but not to stiction that occurs during the operation of the switches. More particularly, the patent describes a method for shaping parts to avoid stiction when the part is fabricated and released.
Other solutions to modifying the restoring force of a micro-electromechanical switch have been described in the patent literature as, for instance, in U.S. Pat. No. 5,901,939 to Cabuz et al., wherein the use of multiple control electrodes and a specially shaped beam to create a stronger restoring force are described. The technique described, however, requires driving multiple electrode pairs in a two-phase configuration, which adds to the cost and complexity of the system employing such a switch. In addition, rather than using a deflecting beam, this switch relies on shifting a buckled region of a metal line toward one end of the line or the other, a technique which generates large flexure of the line and which can generate long-term reliability concerns.
The problem created by stiction during a transition of the switch from the on to the off state has been mainly addressed by investigating such a behavior when it occurs only during the manufacturing process and not during post-production switch operation. Further, most solutions fail in that they do not provide a continued use of a simple, single control electrode or multiple electrodes, all of which are actuated with voltages that are approximately in phase. Moreover, existing solutions fail to introduce an additional restoring force to the deflecting beam by means of a simple electrode coating rather than by employing a new type of beam that is difficult to manufacture and which is normally energized by buckling rather than by deflection (thus introducing high material stresses which have reliability implications).
Thus, it is an object of the invention to provide self-aligned spacers or bumps such that they can act as a detent mechanism for some MEMS actuators.
It is another object to fabricate the self-aligned spacers by making them of the correct size, and further, to position them in the right places.
It is a further object to fabricate the spacers by way of standard semiconductor techniques, particularly, by fabrication methods that are typically used for the manufacture of CMOS devices.
It is still another object to ensure that the fabrication of the spacers requires no added depositions, no extra lithography steps, and no additional etching.
It is yet a further object to minimize the problem caused by stiction in MEMS devices.
To achieve these and other objects, and in view of its purposes, the invention provides self-aligned stops or spacers that are generated during the course of fabricating a MEMS device with no additional processes required. This is based on pre-forming the topography in a manner that initiates the later coincidental formation of these stops.
In one aspect of the invention, there is further provided a process of forming the self-aligned spacers. A metal damascene layer, preferably made of copper, and which is intended to be used as the lower functioning level of a MEMS device (i.e. switch) is first deposited. The layer is formed by depositing a suitable liner/barrier material such as TaN/Ta and a seed layer of Cu. Then the features are filled by Cu plating and the Cu and liner is then planarized. The copper is then subjected to an etchant. A wet etch consisting of water, acetic acid and hydrogen peroxide is advantageously used. The copper near the perimeters of the features etches faster than the bulk plated areas. This creates a differential topography around all metal features. The extent of the etch plays a role in how big the final spacer is to become. When the next layer of conformal dielectric material (e.g., SiO2) is deposited, the topography of the recessed perimeters is transferred to the top surface. As vias are constructed, the trenches fill with metal. The etch of the cavity for the MEMS device will now be influenced by the self-aligned metal filled trenches. For at least part of the etch, they will mask the corresponding areas below. This allows for at least some of the dielectric material to remain, thus forming the self-aligned spacers.
In another aspect of the invention there is provided a method of forming a micro-electro-mechanical switch (MEMS) that includes the steps of: a) depositing a first dielectric layer on a substrate, the first dielectric layer having a plurality of conductive interconnect lines formed therein; b) wet etching metal to form trenches on the boundaries of the conductive interconnect lines; c) depositing a second dielectric layer through which conductive vias are formed the conductive vias contacting at least one of the plurality of conductive interconnect lines and replicating the trench topographies to the top surface of the second dielectric layer and filling with metal the replicated trench topographies; d) forming a cavity that is etched-out from the second dielectric layer and having the metal filled trench topographies selectively inhibit etching the dielectric underneath the metal trench topographies to form self-aligned spacers; e) filling the cavity with sacrificial material and planarizing the sacrificial material; and f) depositing a third dielectric layer to form a conductive beam with the conductive vias contacting the conductive beam.
Spacers may also be used to prevent shorting of plates in a MEM tunable capacitor. In this case, the dielectric bumps prevent the moveable plate from contacting the stationary plate, greatly extending the tunable range of the capacitor. For optical MEMS, they are also used to limit the tilting movement of a mirror or other membrane.
It is understood that both the foregoing general description and the following detailed description are exemplary, but are not restrictive of the invention.